U.S. patent number 7,072,413 [Application Number 09/881,610] was granted by the patent office on 2006-07-04 for method and apparatus for processing data for transmission in a multi-channel communication system using selective channel inversion.
This patent grant is currently assigned to QUALCOMM, Incorporated. Invention is credited to John W. Ketchum, Jay R. Walton.
United States Patent |
7,072,413 |
Walton , et al. |
July 4, 2006 |
Method and apparatus for processing data for transmission in a
multi-channel communication system using selective channel
inversion
Abstract
Techniques to process data for transmission over multiple
transmission channels. The available transmission channels are
segregated into one or more groups, and the channels in each group
are selected for use for data transmission. Data for each group is
coded and modulated based on a particular coding and modulation
scheme to provide modulation symbols, and the modulation symbols
for each selected channel are weighted based on an assigned weight.
The weighting "inverts" the selected channels such that they
achieve similar received SNRs. With selective channel inversion,
only "good" channels in each group having SNRs at or above a
particular threshold are selected, "bad" channels are not used, and
the total available transmit power for the group is distributed
across the good channels in the group. Improved performance is
achieved by using only good channels in each group and matching
each selected channel's received SNR to the required SNR.
Inventors: |
Walton; Jay R. (Westford,
MA), Ketchum; John W. (Harvard, MA) |
Assignee: |
QUALCOMM, Incorporated (San
Diego, CA)
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Family
ID: |
25378821 |
Appl.
No.: |
09/881,610 |
Filed: |
June 14, 2001 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030112880 A1 |
Jun 19, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09860274 |
May 17, 2001 |
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Current U.S.
Class: |
375/267; 370/437;
370/465; 375/299; 455/102; 455/101; 375/146; 370/329 |
Current CPC
Class: |
H04L
25/03343 (20130101); H04L 5/0044 (20130101); H04W
52/346 (20130101); H04W 52/24 (20130101); H04L
5/06 (20130101); H04B 7/0632 (20130101); H04L
25/0244 (20130101); H04B 7/0615 (20130101); H04L
1/0019 (20130101); H04L 1/0009 (20130101); H04L
25/0204 (20130101); H04L 1/06 (20130101); H04L
5/0023 (20130101); H04L 1/0068 (20130101); H04L
5/006 (20130101); H04B 7/0647 (20130101); H04B
7/0417 (20130101); H04L 1/0015 (20130101); H04L
5/0046 (20130101); H04L 25/03159 (20130101); H04L
5/023 (20130101); H04L 27/2608 (20130101); H04B
7/0439 (20130101); H04W 52/42 (20130101); H04L
2025/03802 (20130101); H04B 7/0891 (20130101); H04L
25/0248 (20130101); H04L 2025/03414 (20130101); H04L
5/0037 (20130101); H04L 2001/0096 (20130101); H04L
5/0091 (20130101); H04L 1/0003 (20130101); H04L
2025/03426 (20130101); H04L 25/0228 (20130101) |
Current International
Class: |
H04L
1/02 (20060101) |
Field of
Search: |
;375/141,146,260,267,295,299
;370/318-323,329,330,341,343,344,437,464,465,478,252
;455/101-103,115,13.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1024607 |
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Jul 2000 |
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EP |
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9809381 |
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May 1998 |
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WO |
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0203557 |
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Oct 2002 |
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WO |
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Primary Examiner: Ha; Dac V.
Attorney, Agent or Firm: Milikovsky; Dmitry R. Minha; Sandip
(Micky) S. Wadsworth; Philip R.
Parent Case Text
CROSS REFERENCE
This application is a continuation-in-part of co-pending U.S.
application Ser. No. 09/860,274, filed May 17, 2001, entitled
"Method and Apparatus for Processing Data For Transmission In A
Multi-Chanel Communication System Using Selective Channel
Inversion."
Claims
What is claimed is:
1. A method for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: determining characteristics of a plurality of
transmission channels available for data transmission; segregating
the plurality of transmission channels into one or more groups of
transmission channels; and for each group of transmission channels,
selecting one or more available transmission channels in the group
based on the determined characteristics and a threshold, and coding
and modulating data for all selected transmission channels in the
group based on a particular common coding and modulation scheme
selected for the group to provide modulation symbols.
2. The method of claim 1, further comprising: for each group of
transmission channels weighting modulation symbols for each
selected transmission channel in the group based on a respective
weight indicative of a transmit power level for the selected
transmission channel and derived based in part on the determined
characteristics of the selected transmission channel.
3. The method of claim 2, wherein the weights for the selected
transmission channels in each group are derived to distribute total
transmit power available for the group among all selected
transmission channels in the group to achieve similar received
signal quality.
4. The method of claim 3, wherein the received signal quality is
estimated by a signal-to-noise-plus-interference ratio (SNR).
5. The method of claim 2, wherein the weight for each selected
transmission channel is further derived based on total transmit
power available for the group in which the transmission channel
belongs.
6. The method of claim 2, wherein the weight for each selected
transmission channel is further derived based on a normalization
factor, which is determined based on the characteristics of the
selected transmission channels.
7. The method of claim 2, further comprising: transmitting the
weighted modulation symbols on the selected transmission
channels.
8. The method of claim 1, wherein the multi-channel communication
system is an orthogonal frequency division modulation (OFDM)
system, and wherein the plurality of available transmission
channels correspond to a plurality of frequency subchannels.
9. The method of claim 1, wherein the multi-channel communication
system is a multiple-input multiple-output (MIMO) communication
system, and wherein the plurality of available transmission
channels correspond to a plurality of spatial subchannels of a MIMO
channel.
10. The method of claim 9, wherein the MIMO communication system
utilizes OFDM, and wherein the plurality of available transmission
channels correspond to a plurality of spatial sub channels of a
plurality of frequency subchannels.
11. The method of claim 10, wherein each group corresponds to a
respective transmit antenna, and wherein the plurality of
transmission channels in each group correspond to a plurality of
frequency subchannels for the corresponding transmit antenna.
12. The method of claim 1, wherein each group is associated with a
respective threshold used to select the available transmission
channels in the group for use.
13. The method of claim 1, wherein the determined characteristics
for the available transmission channels are channel gains.
14. The method of claim 13, wherein, for each group, transmission
channels having power gains greater than or equal to a particular
power gain threshold are selected, and wherein the power gains are
determined based on the channel gains.
15. The method of claim 1, wherein the determined characteristics
for the available transmission channels are received
signal-to-noise-plus-interference ratios (SNRs).
16. The method of claim 15, wherein, for each group, transmission
channels having SNRs greater than or equal to a particular SNR
threshold are selected.
17. The method of claim 1, wherein the threshold for each group is
selected to provide high throughput for the selected transmission
channels in the group.
18. The method of claim 1, wherein the threshold for each group is
selected to provide a highest possible throughput for the available
transmission channels in the group.
19. The method of claim 1, wherein the threshold for each group is
derived based on a particular target received SNR for all selected
transmission channels in the group.
20. A method for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: determining characteristics of a plurality of
transmission channels available for data transmission; selecting
one or more available transmission channels based on the determined
characteristics and a metric; coding data for all selected
transmission channels based on a particular common coding scheme to
provide coded data selected for the transmission channels that were
selected; and modulating the coded data for all selected
transmission channels based on a particular modulation scheme to
provide modulation symbols.
21. The method of claim 20, further comprising: weighting
modulation symbols for each selected transmission channel based on
a respective weight indicative of a transmit power level for the
selected transmission channel.
22. The method of claim 21, wherein the weights for the selected
transmission channels are equal.
23. The method of claim 21, wherein the weights for the selected
transmission channels are unequal.
24. The method of claim 21, wherein the weights for the selected
transmission channels are derived based in part on the determined
characteristics of the selected transmission channel.
25. The method of claim 24, wherein the weights for the selected
transmissions are further derived to distribute total available
transmit power amongst all selected transmission channels to
achieve similar received quality for modulation symbols transmitted
via the selected transmission channels.
26. The method of claim 20, wherein the metric relates to
throughput and wherein the one or more transmission channels are
selected based on the throughput achievable for the selected
transmission channels.
27. A method for transmitting data over multiple transmission
channels in a multi-channel communication system, comprising:
determining characteristics of each of a plurality of transmission
channels available for use for data transmission; segregating the
plurality of available transmission channels into one or more
groups; coding and modulating data for selected ones of the
available transmission channels in each group to provide modulation
symbols; weighting modulation symbols for each selected
transmission channel in each group based on a respective weight
indicative of a transmit power level for the selected transmission
channel and derived based in part on the determined characteristics
of the selected transmission channel; and transmitting the weighted
modulation symbols on the selected transmission channels.
28. The method of claim 27, wherein the multi-channel communication
system is a multiple-input multiple-output (MIMO) that utilizes
orthogonal frequency division modulation (OFVM).
29. The method of claim 28, wherein each group corresponds to a
respective transmit antenna, and wherein the plurality of
transmission channels in each group correspond to a plurality of
frequency subchannels for the corresponding transmit antenna.
30. The method of claim 27, wherein the data for the selected
transmission channels in each group is coded based on a common
coding scheme.
31. The method of claim 30, wherein the common coding scheme is
selected from among a plurality of possible coding schemes.
32. The method of claim 27, wherein the modulation symbols for the
selected transmission channels in each group are derived based on a
common modulation scheme.
33. The method of claim 32, wherein the common modulation scheme is
selected from among a plurality of possible modulation schemes.
34. The method of claim 27, wherein the data for the selected
transmission channels in each group is coded and modulated based on
a common coding and modulation scheme selected for the group.
35. The method of claim 27, further comprising: selecting one or
more of the available transmission channels in each group for use
for data transmission based on the determined characteristics of
the transmission channels and a threshold.
36. The method of claim 35, wherein each group is associated with a
respective threshold.
37. A transmitter unit in a multi-channel communication system,
comprising: a controller configured to receive channel state
information (CSI) indicative of characteristics of a plurality of
transmission channels available for data transmission, segregate
the available transmission channels into a plurality of groups, and
select one or more available transmission channels in each group
for use for data transmission based on the channel characteristics
and a threshold; and a transmit data processor coupled to the
controller and configured to receive, code, and modulate data for
each group based on a particular common coding and modulation
scheme selected for the group to provide modulation symbols, and to
weight modulation symbols for each selected transmission channel
based on a respective weight, wherein each weight is indicative of
a transmit power level for the corresponding selected transmission
channel and is derived based in part on the characteristics of the
selected transmission channel.
38. The transmitter of claim 37, wherein the controller is further
configured to determine a particular threshold for each group based
on the characteristics of the available transmission channels.
39. The transmitter of claim 37, further comprising: a transmit
channel processor coupled to the transmit data processor and
configured to receive and demultiplex the weighted modulation
symbols for the selected transmission channels into a plurality of
streams, one stream for each antenna used to transmitted the
modulation symbols.
40. The transmitter of claim 37, wherein the CSI comprise
signal-to-noise-plus-interference ratio (SNR) estimates for the
available transmission channels.
41. The transmitter of claim 37, wherein the CSI comprise channel
gain estimates for the available transmission channels.
42. A method for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: determining characteristics of a plurality of frequency
subchannels of an orthogonal frequency division modulation (OFDM)
system; selecting a group of frequency subchannels based on the
determined characteristics and a metric; and selecting a common
modulation and coding scheme for the group of frequency
subchannels.
43. The method of claim 42, further comprising: weighting
modulation symbols for each selected subchannel in the group based
on a respective weight indicative of a transmit power level for the
selected subchannel.
44. The method of claim 42, wherein each group is associated with a
respective metric used to select the subchannels in the group for
use.
45. An apparatus for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: means for determining characteristics of a plurality of
frequency subchannels of an orthogonal frequency division
modulation (OFDM) system; means for selecting a group of frequency
subchannels based on the determined characteristics and a metric;
and means for selecting a common modulation and coding scheme for
the group of frequency subchannels.
46. The apparatus of claim 45, further comprising: means for
weighting modulation symbols for each selected subchannel in the
group based on a respective weight indicative of a transmit power
level for the selected subchannel.
47. The apparatus of claim 45, wherein each group is associated
with a respective metric used to select the subchannels in the
group for use.
48. A method for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: determining characteristics of a plurality of spatial
channels of a multiple-input multiple-output (MIMO) communication
system; selecting a group of spatial channels based on the
determined characteristics and a metric; and selecting a common
modulation and coding scheme for the group of spatial channels.
49. The method of claim 48, further comprising: weighting
modulation symbols for each selected spatial channels in the group
based on a respective weight indicative of a transmit power level
for the selected spatial channels.
50. The method of claim 48, wherein each group is associated with a
respective metric used to select the spatial channels in the group
for use.
51. An apparatus for processing data for transmission over multiple
transmission channels in a multi-channel communication system,
comprising: means for determining characteristics of a plurality of
spatial channels of a multiple-input multiple-output (MIMO)
communication system; means for selecting a group of spatial
channels based on the determined characteristics and a metric; and
means for selecting a common modulation and coding scheme for the
group of spatial channels.
52. The apparatus of claim 51, further comprising: means for
weighting modulation symbols for each selected spatial channel in
the group based on a respective weight indicative of a transmit
power level for the selected spatial channel.
53. The apparatus of claim 51, wherein each group is associated
with a respective threshold used to select the spatial channels in
the group for use.
Description
BACKGROUND
1. Field
The present invention relates generally to data communication, and
more specifically to a novel and improved method and apparatus for
processing data for transmission in a wireless communication system
using selective channel inversion.
2. Background
A multi-channel communication system is often deployed to provide
increased transmission capacity for various types of communication
such as voice, data, and so on. Such a multi-channel system may be
a multiple-input multiple-output (MIMO) communication system, an
orthogonal frequency division modulation (OFDM) system, a MIMO
system that utilizes OFDM, or some other type of system. A MIMO
system employs multiple transmit antennas and multiple receive
antennas to exploit spatial diversity to support a number of
spatial subchannels, each of which may be used to transmit data. An
OFDM system effectively partitions the operating frequency band
into a number of frequency subchannels (or frequency bins), each of
which is associated with a respective subcarrier on which data may
be modulated. A multi-channel communication system thus supports a
number of "transmission" channels, each of which may correspond to
a spatial subchannel in a MIMO system, a frequency subchannel in an
OFDM system, or a spatial subchannel of a frequency subchannel in a
MIMO system that utilizes OFDM.
The transmission channels of a multi-channel communication system
typically experience different link conditions (e.g., due to
different fading and multipath effects) and may achieve different
signal-to-noise-plus-interference ratios (SNRs). Consequently, the
transmission capacities (i.e., the information bit rates) that may
be supported by the transmission channels for a particular level of
performance may be different from channel to channel. Moreover, the
link conditions typically vary over time. As a result, the bit
rates supported by the transmission channels also vary with
time.
The different transmission capacities of the transmission channels
plus the time-variant nature of these capacities make it
challenging to provide an effective coding and modulation scheme
capable of processing data prior to transmission on the channels.
Moreover, for practical considerations, the coding and modulation
scheme should be simple to implement and utilize at both the
transmitter and receiver systems.
There is therefore a need in the art for techniques to effectively
and efficiently process data for transmission on multiple
transmission channels with different capacities.
SUMMARY
Aspects of the invention provide techniques to process data for
transmission over multiple transmission channels selected from
among all available transmission channels. The available
transmission channels (e.g., the spatial subchannels and frequency
subchannels in a MIMO system that utilizes OFDM) are segregated
into one or more groups, with each group including any number of
transmission channels. In an aspect, the data processing includes
coding and modulating data for each group based on a common coding
and modulation scheme selected for that group to provide modulation
symbols and weighting the modulation symbols for each selected
transmission channel based on a weight assigned to the channel. The
weighting effectively "inverts" the selected transmission channels
in each group such that these channels achieve approximately
similar received signal-to-noise-plus-interference ratios
(SNRs).
In one embodiment, which is referred to as selective channel
inversion (SCI), only "good" transmission channels in each group
having SNRs (or power gains) at or above a particular (SNR or power
gain) threshold are selected for use for data transmission, and
"bad" transmission channels are not used. With selective channel
inversion, the total available transmit power for each group is
distributed (unevenly) across the good transmission channels, and
improved efficiency and performance are achieved. In another
embodiment, all available transmission channels in each group are
selected for use and the channel inversion is performed for all
available channels in the group.
Each group of transmission channels may be associated with (1) a
respective (SNR or power gain) threshold used to select
transmission channels for use for data transmission and (2) a
respective coding and modulation scheme used to code and modulate
the data for the group. For a MIMO system that utilizes OFDM, each
group may correspond to a respective transmit antenna, and the
transmission channels in each group may be the frequency
subchannels for the corresponding transmit antenna.
The channel inversion techniques simplify the coding/modulation at
a transmitter system and the decoding/demodulation at a receiver
system. Moreover, the selective channel inversion technique may
also provide improved performance due to the combined benefits of
(1) using only the N.sub.S best transmission channels in each group
selected from among all available transmission channels in the
group and (2) matching the received SNR of each selected
transmission channel to the SNR required by the coding and
modulation scheme used for the group in which the channel
belongs.
The invention further provides methods, systems, and apparatus that
implement various aspects, embodiments, and features of the
invention, as described in further detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, nature, and advantages of the present invention will
become more apparent from the detailed description set forth below
when taken in conjunction with the drawings in which like reference
characters identify correspondingly throughout and wherein:
FIG. 1 is a diagram of a multiple-input multiple-output (MIMO)
communication system that may be designed and operated to implement
various aspects and embodiments of the invention;
FIG. 2A is a flow diagram of a process to determine the amount of
transmit power to be allocated to each selected transmission
channel based on selective channel inversion, in accordance with an
embodiment of the invention;
FIG. 2B is a flow diagram of a process to determine a threshold a
used to select transmission channels for data transmission, in
accordance with an embodiment of the invention;
FIG. 3 is a diagram of a MIMO communication system capable of
implementing various aspects and embodiments of the invention;
FIGS. 4A through 4D are block diagrams of four MIMO transmitter
systems capable of processing data in accordance with four specific
embodiments of the invention;
FIG. 5 is a block diagrams of a MIMO receiver system capable of
receiving data in accordance with an embodiment of the
invention;
FIGS. 6A and 6B are block diagrams of an embodiment of a channel
MIMO/data processor and an interference canceller, respectively,
within the MIMO receiver system shown in FIG. 5; and
FIG. 7 is a block diagram of a MIMO receiver system capable of
receiving data in accordance with another embodiment of the
invention.
DETAILED DESCRIPTION
Various aspects, embodiments, and features of the invention may be
applied to any multi-channel communication system in which multiple
transmission channels are available for data transmission. Such
multi-channel communication systems include multiple-input
multiple-output (MIMO) systems, orthogonal frequency division
modulation (OFDM) systems, MIMO systems that utilize OFDM, and
others. The multi-channel communication systems may also implement
code division multiple access (CDMA), time division multiple access
(TDMA), frequency division multiple access (FDMA), or some other
multiple access techniques. Multiple access communication systems
can support concurrent communication with a number of terminals
(i.e., users).
FIG. 1 is a diagram of a multiple-input multiple-output (MIMO)
communication system 100 that may be designed and operated to
implement various aspects and embodiments of the invention. MIMO
system 100 employs multiple (N.sub.T) transmit antennas and
multiple (N.sub.R) receive antennas for data transmission. MIMO
system 100 is effectively formed for a multiple access
communication system having a base station (BS) 104 that
concurrently communicates with a number of terminals (T) 106. In
this case, base station 104 employs multiple antennas and
represents the multiple-input (MI) for uplink transmissions and the
multiple-output (MO) for downlink transmissions. The downlink
(i.e., forward link) refers to transmissions from the base station
to the terminals, and the uplink (i.e., reverse link) refers to
transmissions from the terminals to the base station.
A MIMO system employs multiple (N.sub.T) transmit antennas and
multiple (N.sub.R) receive antennas for data transmission. A MIMO
channel formed by the N.sub.T transmit and N.sub.R receive antennas
may be decomposed into N.sub.C independent channels, with
N.sub.C.ltoreq.min {N.sub.T, N.sub.R}. Each of the N.sub.C
independent channels is also referred to as a spatial subchannel of
the MIMO channel and corresponds to a dimension. In one common MIMO
system implementation, the N.sub.T transmit antennas are located at
and associated with a single transmitter system, and the N.sub.R
receive antennas are similarly located at and associated with a
single receiver system. A MIMO system may also be effectively
formed for a multiple access communication system having a base
station that concurrently communicates with a number of terminals.
In this case, the base station is equipped with a number of
antennas and each terminal may be equipped with one or more
antennas.
An OFDM system effectively partitions the operating frequency band
into a number of (N.sub.F) frequency subchannels (i.e., frequency
bins or subbands). At each time slot, a modulation symbol may be
transmitted on each of the N.sub.F frequency subchannels. Each time
slot corresponds to a particular time interval that may be
dependent on the bandwidth of the frequency subchannel.
A multi-channel communication system may be operated to transmit
data via a number of transmission channels. For a MIMO system not
utilizing OFDM, there is typically only one frequency subchannel
and each spatial subchannel may be referred to as a transmission
channel. For a MIMO system utilizing OFDM, each spatial subchannel
of each frequency subchannel may be referred to as a transmission
channel. And for an OFDM system not utilizing MIMO, there is only
one spatial subchannel for each frequency subchannel and each
frequency subchannel may be referred to as a transmission
channel.
The transmission channels in a multi-channel communication system
typically experience different link conditions (e.g., due to
different fading and multipath effects) and may achieve different
signal-to-noise-plus-interference ratios (SNRs). Consequently, the
capacity of the transmission channels may be different from channel
to channel. This capacity may be quantified by the information bit
rate (i.e., the number of information bits per modulation symbol)
that may be transmitted on a transmission channel for a particular
level of performance (e.g., a particular bit error rate (BER) or
packet error rate (PER)). Since the link conditions typically vary
with time, the supported information bit rates for the transmission
channels also vary with time.
To more fully utilize the capacity of the transmission channels,
channel state information (CSI) descriptive of the link conditions
may be determined (typically at the receiver system) and provided
to the transmitter system. The transmitter system may then process
(e.g., encode, modulate, and weight) data such that the transmitted
information bit rate for each transmission channel matches the
transmission capacity of the channel. CSI may be categorized as
either "full CSI" or "partial CSI". Full CSI includes sufficient
characterization (e.g., the amplitude and phase) across the entire
system bandwidth for the propagation path between each
transmit-receive antenna pair in a N.sub.T.times.N.sub.R MIMO
matrix (i.e., the characterization for each transmission channel).
Partial CSI may include, for example, the SNRs of the transmission
channels.
Various techniques may be used to process data prior to
transmission over multiple transmission channels. In one technique,
data for each transmission channel may be coded and modulated based
on a particular coding and modulation scheme selected for that
channel based on the channel's CSI. By coding and modulating
separately for each transmission channel, the coding and modulation
may be optimized for the SNR achieved by each channel. In one
implementation of such a technique, a fixed base code is used to
encode data, and the coded bits for each transmission channel are
then punctured (i.e., selectively deleted) to obtain a code rate
supported by that channel. In this implementation, the modulation
scheme for each transmission channel is also selected based on the
channel's code rate and SNR. This coding and modulation scheme is
described in further detail in U.S. patent application Ser. No.
09/776,075, entitled "CODING SCHEME FOR A WIRELESS COMMUNICATION
SYSTEM," filed Feb. 1, 2001, assigned to the assignee of the
present application and incorporated herein by reference. For this
technique, substantial implementation complexity is typically
associated with having a different code rate and modulation scheme
for each transmission channel.
In accordance with an aspect of the invention, techniques are
provided to (1) process data for all selected transmission channels
based on a common coding and modulation scheme to provide
modulation symbols, and (2) weight the modulation symbols for each
selected transmission channel based on the channel's CSI. The
weighting effectively "inverts" the selected transmission channels
such that, in general, the SNRs are approximately similar at the
receiver system for all selected transmission channels. In one
embodiment, which is referred to as selective channel inversion
(SCI), only "good" transmission channels having SNRs (or power
gains) at or above a particular SNR (or power gain) threshold are
selected for use for data transmission, and "bad" transmission
channels are not used. With selective channel inversion, the total
available transmit power is distributed across the good
transmission channels, and improved efficiency and performance are
achieved. In another embodiment, all available transmission
channels are selected for use and the channel inversion is
performed for all transmission channels.
In yet another embodiment, the available transmission channels are
segregated into groups and the selective channel inversion is
applied independently to each group of channels. For example, the
frequency subchannels of each transmit antenna may be grouped
together, and the selective channel inversion may be applied
independently for each of the transmit antennas. This segregation
permits the optimization to be achieved on a per group (e.g., per
transmit antenna) basis.
These channel inversion techniques may be advantageously used when
full or partial CSI is available at the transmitter. These
techniques ameliorate most of the complexity associated with the
channel-specific coding and modulation technique described above,
while still achieving high performance. Moreover, the selective
channel inversion technique may also provide improved performance
over the channel-specific coding and modulation technique due to
the combined benefits of (1) using only the N.sub.S best
transmission channels from among the available transmission
channels and (2) matching the received SNR of each selected
transmission channel to the SNR required for the selected coding
and modulation scheme.
For a MIMO system utilizing OFDM and having full CSI available, the
transmitter system may have knowledge of the complex-valued gain of
the transmission path between each transmit-receive antenna pair of
each frequency subchannel. This information may be used to render
the MIMO channel orthogonal so that each eigenmode (i.e., spatial
subchannel) may be used for an independent data stream.
For a MIMO system utilizing OFDM and having partial CSI available,
the transmitter may have limited knowledge of the transmission
channels. Independent data streams may be transmitted on
corresponding transmission channels over the available transmit
antennas, and the receiver system may use a particular linear
(spatial) or non-linear (space-time) processing technique (i.e.,
equalization) to separate out the data streams. The equalization
provides an independent data stream corresponding to each
transmission channel (e.g., each transmit antenna and/or each
frequency subchannel), and each of these data streams has an
associated SNR.
If the set of SNRs for the transmission channels is available at
the transmitter system, this information may be used to select the
proper coding and modulation scheme and to distribute the total
available transmit power for each group (there may be only one
group). In an embodiment, the available transmission channels in
each group are ranked in order of decreasing received SNR, and the
total available transmit power is allocated to and used for the
N.sub.S best transmission channels in the group. In an embodiment,
transmission channels having received SNRs that fall below a
particular SNR threshold are not selected for use. The SNR
threshold may be selected to optimize throughput or some other
criteria. The total available transmit power for each group is
distributed across all transmission channels in the group selected
for use such that the transmitted data streams have approximately
similar received SNRs at the receiver system. Similar processing
may be performed if the channel gains are available at the
transmitter system. In an embodiment, a common coding scheme (e.g.,
a particular Turbo code of a particular code rate) and a common
modulation scheme (e.g., a particular PSK or QAM constellation) are
used for all selected transmission channels in each group.
Transmission Channel Inversion
If a simple (common) coding and modulation scheme can be used at
the transmitter system, then a single (e.g., convolutional or
Turbo) coder and code rate may be used to encode data for all
transmission channels selected for data transmission, and the
resultant coded bits may be mapped to modulation symbols using a
single (e.g., PSK or QAM) modulation scheme. The resultant
modulation symbols are then all drawn from the same "alphabet" of
possible modulation symbols and encoded with the same code and code
rate. This would then simplify the data processing at both the
transmitter and receiver.
However, the transmission channels in a multi-channel communication
system typically experience different link conditions and achieve
different SNRs. In this case, if the same amount of transmit power
is used for each selected transmission channel, then the
transmitted modulation symbols will be received at different SNRs
depending on the specific channels on which the modulation symbols
are transmitted. The result may be a large variation in symbol
error probability over the set of selected transmission channels,
and an associated loss in bandwidth efficiency.
In accordance with an aspect of the invention, a power control
mechanism is used to set or adjust the transmit power level for
each transmission channel selected for data transmission to achieve
a particular SNR at the receiver system. By achieving similar
received SNRs for all selected transmission channels, a single
coding and modulation scheme may be used for all selected
transmission channels, which can greatly reduce the complexity of
the coding/modulation process at the transmitter system and the
complementary demodulation/decoding process at the receiver system.
The power control may be achieved by "inverting" the selected
transmission channels and properly distributing the total available
transmit power across all selected channels, as described in
further detail below.
If the same amount of transmit power is used for all available
transmission channels in a MIMO system utilizing OFDM, then the
received power for a particular channel may be expressed as:
'.function..times..times..function..times..times. ##EQU00001##
where P'.sub.rx (j,k) is the received power for transmission
channel (j,k) (i.e., the j-th spatial subchannel of the k-th
frequency subchannel), P.sub.tx is the total transmit power
available at the transmitter, N.sub.T is the number of transmit
antennas, N.sub.F is the number of frequency subchannels, and
H(j,k) is the complex-valued "effective" channel gain from the
transmitter to the receiver for transmission channel (j,k). For
simplicity, the channel gain H(j,k) includes the effects of the
processing at the transmitter and receiver. Also for simplicity, it
is assumed that the number of spatial subchannels is equal to the
number of transmit antennas and N.sub.TN.sub.F represents the total
number of available transmission channels. If the same amount of
power is transmitted for each available transmission channel, the
total received power P.sub.rx.sub.--.sub.total for all available
transmission channels may be expressed as:
.times..times..times..times..function..times..times.
##EQU00002##
Equation (1) shows that the receive power for each transmission
channel is dependent on the power gain of that channel, i.e.,
|H(j,k)|.sup.2. To achieve equal received power across all
available transmission channels, the modulation symbols for each
channel can be weighted at the transmitter by a weight of W(j,k),
which can be expressed as:
.function..function..times..times. ##EQU00003## where c is a factor
chosen such that the received powers for all transmission channels
are approximately equal at the receiver. As shown in equation (3),
the weight for each transmission channel is inversely proportional
to that channel's gain. The weighted transmit power for
transmission channel (j,k) can then be expressed as:
.function..function..times..times. ##EQU00004## where b is a
"normalization" factor used to distribute the total transmit power
among the available transmission channels. This normalization
factor b can be expressed as:
.times..times..function..times..times. ##EQU00005## where
c.sup.2=b. As shown in equation (5), the normalization factor b is
computed as the sum of the reciprocal power gains for all available
transmission channels.
The weighting of the modulation symbols for each transmission
channel by W(j,k) effectively "inverts" the transmission channel.
This channel inversion results in the amount of transmit power for
each transmission channel being inversely proportional to the
channel's power gain, as shown in equation (4), which then provides
a particular received power at the receiver. The total available
transmit power is thus effectively distributed (unevenly) to all
available transmission channels based on their channel gains such
that all transmission channels have approximately equal received
power, which may be expressed as: P.sub.rx(j,k)=bP.sub.tx. Eq (6)
If the noise variance is the same across all transmission channels,
then the equal received power allows the modulation symbols for all
channels to be generated based on a single common coding and
modulation scheme, which then greatly simplify the coding and
decoding processes.
If all available transmission channels are used for data
transmission regardless of their channel gains, then the poor
transmission channels are allocated more of the total transmit
power. In fact, to achieve similar received power for all
transmission channels, the poorer a transmission channel gets the
more transmit power needs to be allocated to this channel. When one
or more transmission channels become excessively poor, the amount
of transmit power needed for these channels would deprive (or
starve) the good channels of power, which may then dramatically
decrease the overall system throughput.
Selective Channel Inversion Based on Channel Gains
In an aspect, the channel inversion is applied selectively, and
only transmission channels whose received power is at or above a
particular threshold, .alpha., relative to the total received power
are selected for data transmission. Transmission channels whose
received power falls below this threshold are erased (i.e., not
used). For each selected transmission channel, the modulation
symbols are weighted at the transmitter such that all selected
transmission channels are received at approximately similar power
level. The threshold can be selected to maximize throughput or
based on some other criteria. The selective channel inversion
scheme preserves most of the simplicity inherent in using a common
coding and modulation scheme for all transmission channels while
also provides high performance normally associated with individual
coding per transmission channel.
Initially, the average power gain, L.sub.ave, is computed for all
available transmission channels and can be expressed as:
.times..times..function..times..times..times. ##EQU00006##
The modulation symbols for each selected transmission channel can
be weighted at the transmitter by a weight of {tilde over
(W)}(j,k), which can be expressed as:
.function..function..times..times. ##EQU00007## The weight for each
selected transmission channel is inversely proportional to that
channel's gain and is determined such that all selected
transmission channels are received at approximately equal power.
The weighted transmit power for each transmission channel can then
be expressed as:
.function..times..function..function..gtoreq..alpha..times..times..times.-
.times. ##EQU00008## where .alpha. is the threshold and {tilde over
(b)} is a normalization factor used to distribute the total
transmit power among the selected transmission channels. As shown
in equation (9), a transmission channel is selected for use if its
power gain is greater than or equal to a power gain threshold
(i.e., |H(j,k)|.sup.2.gtoreq..alpha.L.sub.ave). The normalization
factor {tilde over (b)} is computed based on only the selected
transmission channels and can be expressed as:
.function..gtoreq..alpha..times..times..times..function..times..times.
##EQU00009##
Equations (7) through (10) effectively distribute the total
transmit power to the selected transmission channels based on their
power gains such that all selected transmission channels have
approximately equal received power, which may be expressed as:
.function..times..function..gtoreq..alpha..times..times..times..times.
##EQU00010##
Selective Channel Inversion Based on Channel SNRs
In many communication systems, the known quantities at the receiver
system are the received SNRs for the transmission channels rather
than the channel gains (i.e., the path losses). In such systems,
the selective channel inversion technique can be readily modified
to operate based on the received SNRs instead of the channel
gains.
If equal transmit power is used for all available transmission
channels and the noise variance, .sigma..sup.2, is constant for all
channels, then the received SNR, .gamma.(j,k), for transmission
channel (j,k) can be expressed as:
.gamma..function..function..sigma..sigma..times..times..times..function..-
times..times. ##EQU00011## The average received SNR,
.gamma..sub.ave, for each available transmission channel may be
expressed as:
.gamma..sigma..function..times..times..times..times..function..times..tim-
es. ##EQU00012## which also assumes equal transmit power over the
available transmission channels. The received SNR,
.gamma..sub.total, for all available transmission channels may be
expressed as:
.gamma..sigma..times..sigma..times..times..times..times..times..function.-
.times..times. ##EQU00013## The total received SNR,
.gamma..sub.total, is based on the total transmit power being
equally distributed across all available transmission channels.
A normalization factor, .beta., used to distribute the total
transmit power among the selected transmission channels can be
expressed as:
.beta..gamma..function..gtoreq..alpha..gamma..times..gamma..function..tim-
es..times. ##EQU00014## As shown in equation (15), the
normalization factor .beta. is computed based on, and as the sum of
the reciprocal of, the SNRs of all selected transmission
channels.
To achieve similar received SNR for all selected transmission
channels, the modulation symbols for each selected transmission
channel (j,k) may be weighted by a weight that is related to that
channel's SNR, which may be expressed as:
.function..gamma..function..times..times. ##EQU00015## where {tilde
over (c)}.sup.2=.beta.. The weighted transmit power for each
transmission channel may then be expressed as:
.function..beta..times..times..gamma..function..gamma..function..gtoreq..-
alpha..gamma..times..times. ##EQU00016## As shown in equation (17),
only transmission channels for which the received SNR is greater
than or equal to an SNR threshold (i.e.,
.gamma.(j,k).gtoreq..alpha..gamma..sub.ave) are selected for
use.
If the total transmit power is distributed across all selected
transmission channels such that the received SNR is approximately
similar for all selected channels, then the resulting received SNR
for each transmission channel may be expressed as:
.gamma..function..beta..gamma..gamma..gamma..function..gtoreq..alpha..gam-
ma..times..times. ##EQU00017## By substituting .gamma..sub.ave from
equation (13) and .gamma..sub.total from equation (14) into
equation (18), the following is obtained:
.gamma..function..beta..times..times..times..gamma..function..gtoreq..alp-
ha..gamma. ##EQU00018##
Channel Inversion for Segregated Groups of Transmission
Channels
In the above description, the channel inversion is applied to all
available transmission channels or selectively to a subset of the
available transmission channels (which are selected based on a
particular threshold). This then allows a common coding and
modulation scheme to be used for all transmission channels to be
used for data transmission.
The selective channel inversion may also be applied individually
and independently to groups of transmission channels. In this case,
the available transmission channels in the communication system are
initially segregated into a number of groups. Any number of groups
may be formed, and each group may include any number of channels
(i.e., there need not be equal number of channels in each
group).
A particular amount of transmit power is also available for each
group based on various system constraints and considerations. For a
full channel inversion technique, the available transmit power for
each group is allocated to all transmission channels in the group
such that the received signal quality for these channels is
approximately equal (i.e., similar received SNRs). And for a
selective channel inversion technique, all or a subset of the
available transmission channels in each group are selected for use,
e.g., based on a particular threshold determined for the group. The
available transmit power for each group is then allocated to the
selected transmission channels in the group such that the received
signal quality for the channels is approximately equal.
Various additional flexibilities are afforded by processing data
separately for each group of transmission channels. For example,
the full or selective channel inversion may be independently
applied to each group of channels. Also, for those groups for which
selective channel inversion is applied, one threshold may be used
for all groups, each group may be assigned a separate threshold, or
some groups may share the same threshold while other groups may be
assigned separate thresholds. A different coding and modulation
scheme may also be used for each group, which may be selected based
on the received SNR achieved by the transmission channels in the
group.
For a MIMO system that utilizes OFDM, the MIMO construct creates
multiple (N.sub.S) transmission channels in the spatial domain and
the OFDM construct creates multiple (N.sub.F) transmission channels
in the frequency domain. The total number of transmission channels
available to send data is then N=N.sub.SN.sub.F. The N transmission
channels may then be segregated into a number of groups in various
ways.
In one embodiment, the transmission channels are segregated on a
per transmit antenna basis. If the number of spatial subchannels is
equal to the number of transmit antennas (i.e., N.sub.T=N.sub.S),
then the full or selective channel inversion may be applied
independently to each of the N.sub.T transmit antennas. In an
embodiment, selective channel inversion is used for each group, and
the N.sub.T groups corresponding to the N.sub.T transmit antennas
may be associated with N.sub.T respective thresholds, one threshold
for each group or transmit antenna. The selective channel inversion
then determines the subset of transmission channels (or frequency
subchannels) associated with each transmit antenna having adequate
received SNRs, which can be achieved by comparing the received SNR
for each frequency subchannel to the threshold for the transmit
antenna. The total transmit power available for each transmit
antenna is then allocated to the selected frequency subchannels for
the transmit antenna such that the received SNRs for these
frequency subchannels are approximately similar.
In another embodiment, the available transmission channels are
segregated on a per frequency subchannel basis. In this embodiment,
the full or selective channel inversion may be applied
independently to each of the N.sub.F frequency subchannels. If
selective channel inversion is used, then the spatial subchannels
in each group may be selected for use for data transmission based
on the threshold for the group corresponding to that frequency
subchannel.
The segregation of the available transmission channels into groups
permits optimization to be achieved on a per group basis (e.g., per
transmit antenna or per frequency subchannel), which then allows a
specific coding and modulation scheme to be used for all selected
transmission channels in each group. For example, one or more
transmit antennas may be assigned to each scheduled terminal for
data transmission. The transmission channels associated with the
assigned transmit antennas may be placed in a group, and the
selective channel inversion may be performed on this group of
transmission channels such that a single coding and modulation
scheme may be used for the data transmission to this terminal.
If equal transmit power is used for all available transmission
channels in group j and the noise variance, .sigma..sup.2, is
constant for all channels, then the received SNR, .gamma..sub.j(k),
for transmission channel k in group j can be expressed as:
.gamma..function..function..sigma..sigma..times..times..function..times..-
times. ##EQU00019## where P.sub.rx,j(k) is the received power for
transmission channel k in group j, P.sub.tx,j is the total
available transmit power for group j, H.sub.j(k) is effective
channel gain from the transmitter to the receiver for transmission
channel k in group j, and N.sub.j is the number of transmission
channels in group j. Group j may correspond to a specific transmit
antenna j, in which case N.sub.j=N.sub.F. The average received SNR,
.gamma..sub.ave,j, for each available transmission channel in group
j may be expressed as:
.gamma..sigma..times..times..times..function..times..times.
##EQU00020## Equation (20) assumes equal transmit power over the
N.sub.j available transmission channels in group j. The received
SNR, .gamma..sub.total,j, for all available transmission channels
in group may then be expressed as:
.gamma..sigma..times..sigma..times..times..times..function..times..times.-
.times..times..times..function..times..times. ##EQU00021## The
total received SNR, .gamma..sub.total,j, for group j is based on
the total transmit power, P.sub.tx,j, for group j being equally
distributed across all available transmission channels in the
group.
A normalization factor, .beta..sub.j, used to distribute the total
transmit power P.sub.tx,j among the selected transmission channels
in group j can be expressed as:
.beta..gamma..function..gtoreq..alpha..times..gamma..times..gamma..functi-
on..times..times. ##EQU00022## As shown in equation (23), the
normalization factor .beta..sub.j is computed based on the SNRs of
all selected transmission channels in group j, with the channels
being selected based on the threshold,
.alpha..sub.j.gamma..sub.ave,j, determined for the group.
To achieve similar received SNR for all selected transmission
channels in the group, the modulation symbols for each selected
transmission channel may be weighted by a weight that is related to
that channel's SNR, which may be expressed as:
.function..gamma..function..times..times. ##EQU00023## where {tilde
over (c)}.sub.j.sup.2=.beta..sub.j. The weighted transmit power for
each transmission channel may then be expressed as:
.function..beta..times..gamma..function..gamma..function..gtoreq..alpha..-
times..gamma..times..times. ##EQU00024## As shown in equation (25),
only transmission channels for which the received SNR is greater
than or equal to the SNR threshold (i.e.,
.gamma..sub.j(k).gtoreq..alpha..sub.j.gamma..sub.ave,j) are
selected for use.
If the total transmit power is distributed across all selected
transmission channels in the group such that the received SNR is
approximately similar for all selected channels, then the resulting
received SNR for each transmission channel may be expressed as:
.gamma..function..beta..times..gamma..gamma..beta..times..gamma..function-
..gtoreq..alpha..times..gamma..times..times. ##EQU00025##
The process described above may be repeated for each group of
transmission channels. Each group may be associated with a
different threshold, .alpha..sub.j.gamma..sub.ave,j, derived to
provide the desire performance for that group. The ability to
allocate transmit power on a per group (e.g., per transmit antenna)
basis can provide enhanced flexibility and may further improve
performance.
FIG. 2A is a flow diagram of a process 200 to determine the amount
of transmit power to be allocated to each selected transmission
channel based on selective channel inversion, in accordance with an
embodiment of the invention. Process 200 assumes that all available
transmission channels are considered (i.e., one group of
transmission channels for the communication system). Process 200
may be used if the channel gains H(j,k), the received SNRs
.gamma.(j,k), or some other characteristics are available for the
transmission channels. For clarity, process 200 is described below
for the case in which the channel gains are available, and the case
in which the received SNRs are available is shown within
brackets.
Initially, the channel gains H(j,k) [or the received SNRs
.gamma.(j,k)] of all available transmission channels are retrieved,
at step 212. A power gain threshold, .alpha.L.sub.ave, [or an SNR
threshold, .alpha..gamma..sub.ave] used to select transmission
channels for data transmission is also determined, at step 214. The
threshold may be computed as described in further detail below.
Each available transmission channel is then evaluated for possible
use. A (not yet evaluated) available transmission channel is
identified for evaluation, at step 216. For the identified
transmission channel, a determination is made whether or not the
power gain [or the received SNR] for the channel is greater than or
equal to the power gain threshold (i.e.,
|H(j,k)|.sup.2.gtoreq..alpha.L.sub.ave) [or the SNR threshold
(i.e., .gamma.(j,k).gtoreq..alpha..gamma..sub.ave], at step 218. If
the identified transmission channel satisfies the criteria, then it
is selected for use, at step 220. Otherwise, if the transmission
channel does not satisfy the criteria, it is discarded and not used
for data transmission.
A determination is then made whether or not all available
transmission channels have been evaluated, at step 222. If not, the
process returns to step 216 and another available transmission
channel is identified for evaluation. Otherwise, the process
proceeds to step 224.
At step 224, a normalization factor {tilde over (b)} [or .beta.]
used to distribute the total transmit power among the selected
transmission channels is determined based on the channel gains [or
the received SNRs] of the selected channels, at step 224. This can
be achieved as shown in equation (10) [or equation (15)]. A weight
{tilde over (W)}(j,k) is next computed for each selected
transmission channel, at step 226, based on the normalization
factor and that channel's gain [or SNR]. The weight can be computed
as shown in equation (8) [or equation (16)]. The weighted transmit
power for each selected transmission channel would then be as shown
in equation (9) [or equation (17)]. The process then
terminates.
In the above description, the total available transmit power for
each group is allocated (unevenly) to the selected transmission
channels in the group based on their respective weights such that
the received SNRs for these channels are approximately similar.
(There may be only one group of transmission channels.) In some
other embodiments, the total available transmit power may be
allocated equally amongst the selected transmission channels, in
which case the weights for the selected transmission channels are
equal. This may be implemented, for example, if the common coding
and modulation scheme for a group is selected based on the average
SNR for the selected transmission channels in the group. The
desired level of performance may be achieved, for example, by
interleaving the data across all selected transmission channels in
the group or via some other processing scheme.
Threshold Selection
The threshold, .alpha., used to select transmission channels for
use for data transmission may be set based on various criteria. In
one embodiment, the threshold is set to optimize throughput.
Initially, a vector of setpoints (i.e., Z=[z.sub.1, z.sub.2, . . .
z.sub.N.sub.Z]) and a vector of code rates (i.e., R=[r.sub.1,
r.sub.2, . . . , r.sub.N.sub.Z]) are defined. The code rates
include the effects of the coding and modulation scheme and are
representative of the number of information bits per modulation
symbol. Each vector includes N.sub.Z elements corresponding to the
number of available code rates, which may be those available for
use in the system. Alternatively, N.sub.Z setpoints may be defined
based on the operating points supported by the system. Each
setpoint corresponds to a particular received SNR needed to achieve
a particular level of performance. The setpoint is typically
dependent on the transmission bit rate (i.e., the number of
information bits per modulation symbol), which is further dependent
on the code rate and the modulation scheme used for the data
transmission. As noted above, a common modulation scheme is used
for all selected transmission channels. In this case, the
transmission bit rate and thus the setpoint is directly related to
the code rate.
Each code rate r.sub.n, where 1<n<N.sub.Z, is associated with
a respective setpoint z.sub.n, which is the minimum received SNR
required to operate at that code rate for the required level of
performance. The required setpoint z.sub.n may be determined based
on computer simulation, mathematical derivation, and/or empirical
measurement, as is known in the art. The elements in the two
vectors R and Z may also be ordered such that
{z.sub.1>z.sub.2> . . . >z.sub.N.sub.Z} and
{r.sub.1>r.sub.2> . . . >r.sub.N.sub.Z}, with z.sub.1
being the largest setpoint and r.sub.1 being the highest supported
code rate.
The channel gains for all available transmission channels are used
to compute power gains, which are then ranked and placed in a list
H(l) in order of decreasing power gains, where
1.ltoreq.l.ltoreq.N.sub.TN.sub.F, such that
H(1)=max{|H(j,k)|.sup.2}, . . . , and H(N.sub.TN.sub.F)=min
{|H(j,k)|.sup.2}.
A sequence {tilde over (b)}(l) of possible normalization factors is
also defined as follows:
.function..times..function..ltoreq..ltoreq..times..times..times.
##EQU00026## Each element of the sequence {tilde over (b)}(l) may
be used as a normalization factor if the l best transmission
channels are selected for use.
For each code rate r.sub.n (where 1.ltoreq.n.ltoreq.N.sub.Z), the
largest value of l, l.sub.n,max, is determined such that the
received SNR for each of the l best transmission channels is
greater than or equal to the setpoint z.sub.n associated with the
code rate r.sub.n. This condition may be expressed as:
.function..times..sigma..gtoreq..times..times. ##EQU00027## where
.sigma..sup.2 is the received noise power in a single transmission
channel. The largest value of l, l.sub.n,max, can be identified by
evaluating each possible value of l starting with 1 and terminating
when equation (28) is no longer valid. For each value of l, the
achievable SNR for the l best transmission channels may be
determined as shown by the left argument of equation (28). This
achievable SNR is then compared against the SNR, z.sub.n, required
for that code rate r.sub.n.
Thus, for each code rate r.sub.n, each value of l (for l=1, 2, . .
. , l.sub.n,max) is evaluated to determine whether the received SNR
for each of the l best transmission channels can achieve the
associated setpoint z.sub.n, if the total transmit power is
(unevenly) distributed across all l channels. The largest value of
l, l.sub.n,max, that satisfies this condition is the greatest
number of transmission channels that may be selected for code rate
r.sub.n while achieving the required setpoint z.sub.n.
The threshold, .alpha..sub.n, associated with code rate r.sub.n may
then be expressed as:
.alpha..function..times..times..times. ##EQU00028## The threshold
.alpha..sub.n optimizes the throughput for code rate r.sub.n, which
requires the setpoint z.sub.n. Since a common code rate is used for
all selected transmission channels, the maximum achievable
throughput, T.sub.n, can be computed as the throughput for each
channel (which is r.sub.n) times the number of selected channels,
l.sub.n,max. The maximum achievable throughput T.sub.n for setpoint
z.sub.n can then be expressed as: T.sub.n=l.sub.n,maxr.sub.n, Eq
(30) where the unit for T.sub.n is in information bits per
modulation symbol.
The optimum throughput for the vector of setpoints can then be
given by: T.sub.opt=max{T.sub.n}. Eq (31) As the code rate
increases, more information bits may be transmitted per modulation
symbol. However, the required SNR also increases, which requires
more transmit power for each selected transmission channel for a
given noise variance .sigma..sup.2. Since the total transmit power
is limited, fewer transmission channels may be able to achieve the
higher required SNR. Thus, the maximum achievable throughput for
each code rate in the vector R may be computed, and the specific
code rate that provides the highest throughput may be deemed as the
optimum code rate for the specific channel conditions being
evaluated. The optimum threshold, .alpha..sub.opt, is then equal to
the threshold .alpha..sub.n corresponding to the specific code rate
r.sub.n that results in T.sub.opt.
In the above description, the optimum threshold .alpha..sub.opt is
determined based on the channel gains for all transmission
channels. If the received SNRs are available instead of the channel
gains, then the received SNRs may be ranked and placed in a list
.gamma.(l) in order of decreasing SNRs, where
1.ltoreq.l.ltoreq.N.sub.TN.sub.F, such that the first element in
the list .gamma.(1)=max {.gamma.(j,k)}, . . . , and the last
element in the list .gamma.(N.sub.TN.sub.R)=min {.gamma.(j,k)}. A
sequence .beta.(l) may then be determined as:
.beta..function..times..gamma..function..times..times.
##EQU00029##
For each code rate r.sub.n (where 1.ltoreq.n.ltoreq.N.sub.Z), the
largest value of l, l.sub.n,max, is determined such that the
received SNR for each of the l selected transmission channels is
greater than or equal to the associated setpoint z.sub.n. This
condition may be expressed as:
.beta.(l)N.sub.TN.sub.F.gtoreq.z.sub.n. Eq (33) Once the largest
value of l, l.sub.n,max, is determined for code rate r.sub.n, the
threshold .alpha..sub.n associated with this code rate may be
determined as:
.alpha..gamma..function..gamma..times..times. ##EQU00030## The
optimum threshold, .alpha..sub.opt, and the optimum throughput,
T.sub.opt, may also be determined as described above.
For the above description, the threshold is selected to optimize
throughput for the available transmission channels. The threshold
may also be selected to optimize other performance criteria or
metrics, and this is within the scope of the invention.
FIG. 2B is a flow diagram of a process 240 to determine a threshold
a used to select transmission channels for data transmission, in
accordance with an embodiment of the invention. Process 240 may be
used if the channel gains, received SNRs, or some other
characteristics are available for the transmission channels. For
clarity, process 240 is described below for the case in which the
channel gains are available, and the case in which the received
SNRs are available is shown within brackets.
Initially, a vector of setpoints (Z=[z.sub.1, z.sub.2, . . . ,
z.sub.N.sub.Z]) is defined and a vector of code rates (R=[r.sub.1,
r.sub.2, . . . , r.sub.N.sub.Z]) that supports the associated
setpoints is determined, at step 250. The channel gains H(j,k) [or
the received SNRs .gamma.(j,k)] for all available transmission
channels are retrieved and ranked from the best to the worst, at
step 252. The sequence {tilde over (b)}(l) [or .beta.(l)] of
possible normalization factors is then determined based on the
channel gains as shown in equation (27) [or based on the received
SNRs as shown in equation (32)], at step 254.
Each available code rate is then evaluated via a loop. In the first
step of the loop, a (not yet evaluated) code rate r.sub.n is
identified for evaluation, at step 256. For the first pass through
the loop, the identified code rate can be the first code rate
r.sub.1 in the vector R. For the identified code rate r.sub.n, the
largest value of l, l.sub.n,max, is determined such that the
received SNR for each of the l best transmission channels is
greater than or equal to the setpoint z.sub.n associated with the
code rate r.sub.n being evaluated, at step 258. This can be
performed by computing and satisfying the condition shown in
equation (28) [or equation (33)]. The threshold .alpha..sub.n
associated with setpoint z.sub.n is then determined based on the
channel gain [or the received SNR] of channel l.sub.n,max as shown
in equation (29) [or equation (34)], at step 260. The maximum
achievable throughput, T.sub.n, for setpoint z.sub.n can also be
determined as shown in equation (30), at step 262.
A determination is then made whether or not all N.sub.Z code rates
have been evaluated, at step 264. If not, the process returns to
step 256 and another code rate is identified for evaluation.
Otherwise, the optimum throughput, T.sub.opt, and the optimum
threshold, .alpha..sub.opt, may be determined as shown in equation
(31), at step 266. The process then terminates.
In the above description, one threshold is determined for all
available transmission channels in the communication system since
the selective channel inversion is performed on all channels. In
embodiments wherein the transmission channels are segregated into a
number of groups, one threshold may be determined and used for each
group. The threshold for each group may be set based on various
criteria, such as to optimize the throughput for the transmission
channels included in the group.
To determine the threshold for each group, the derivations
described above may also be used. However, the list H.sub.j(l) [or
.gamma..sub.j(l)] for each group only includes the power gains [or
received SNRs] for the transmission channels included in the group.
Also, the sequence {tilde over (b)}.sub.j(l) [or .beta..sub.j(l)]
would include the possible normalization factors defined based on
the channel gains [or received SNRs] of the transmission channels
in the group. The threshold .alpha..sub.j,n associated with code
rate r.sub.n for group j may then be expressed as:
.alpha..function..times..times..times..times..gamma..function..gamma..tim-
es..times. ##EQU00031## The optimum threshold .alpha..sub.opt,j for
group j is equal to the threshold .alpha..sub.j,n corresponding to
the specific code rate r.sub.n that results in the optimal
throughput T.sub.opt,j for group j.
Each group of transmission channels may be associated with a
respective threshold. Alternatively, a number of groups may share
the same threshold. This may be desirable, for example, if the same
coding and modulation scheme is to be used for a number of transmit
antennas and the available transmit power may be shared between
these transmit antennas.
In the above description, the threshold is derived based on
(unequal) distribution of the total available transmit power
amongst the selected transmission channels to achieve similar
received SNRs for these channels. In some other embodiments, the
threshold may be derived based on some other conditions and/or
metrics. For example, the threshold may be derived based on equal
allocation of the total available transmit power amongst the
selected transmission channels (i.e., equal weights for the
selected transmission channels). In this case, the threshold may be
selected to maximize the throughput achieved based on this equal
transmit power allocation. As another example, the threshold may
simply be a particular (fixed) target SNR.
Multi-Channel Communication System
FIG. 3 is a diagram of a MIMO communication system 300 capable of
implementing various aspects and embodiments of the invention.
System 300 includes a first system 310 (e.g., base station 104 in
FIG. 1) in communication with a second system 350 (e.g., terminal
106). System 300 may be operated to employ a combination of
antenna, frequency, and temporal diversity to increase spectral
efficiency, improve performance, and enhance flexibility.
At system 310, a data source 312 provides data (i.e., information
bits) to a transmit (TX) data processor 314, which (1) encodes the
data in accordance with a particular encoding scheme, (2)
interleaves (i.e., reorders) the encoded data based on a particular
interleaving scheme, (3) maps the interleaved bits into modulation
symbols for one or more transmission channels selected for use for
data transmission, and (4) weights the modulation symbols for each
selected transmission channel. The encoding increases the
reliability of the data transmission. The interleaving provides
time diversity for the coded bits, permits the data to be
transmitted based on an average SNR for the selected transmission
channels, combats fading, and further removes correlation between
coded bits used to form each modulation symbol. The interleaving
may further provide frequency diversity if the coded bits are
transmitted over multiple frequency subchannels. The weighting
effectively controls the transmit power for each selected
transmission channel to achieve a desired SNR at the receiver
system. In an aspect, the coding, symbol mapping, and weighting may
be performed based on control signals provided by a controller
334.
A TX channel processor 320 receives and demultiplexes the weighted
modulation symbols from TX data processor 314 and provides a stream
of weighted modulation symbols for each selected transmission
channel, one weighted modulation symbol per time slot. TX channel
processor 320 may further precondition the weighted modulation
symbols for the selected transmission channels if full CSI is
available.
If OFDM is not employed, TX channel processor 320 provides a stream
of weighted modulation symbols for each antenna used for data
transmission. And if OFDM is employed, TX channel processor 320
provides a stream of weighted modulation symbol vectors for each
antenna used for data transmission. And if full-CSI processing is
performed, TX channel processor 320 provides a stream of
preconditioned modulation symbols or preconditioned modulation
symbol vectors for each antenna used for data transmission. Each
stream is then received and modulated by a respective modulator
(MOD) 322 and transmitted via an associated antenna 324.
At receiver system 350, a number of receive antennas 352 receive
the transmitted signals and provide the received signals to
respective demodulators (DEMOD) 354. Each demodulator 354 performs
processing complementary to that performed at modulator 322. The
modulation symbols from all demodulators 354 are provided to a
receive (RX) channel/data processor 356 and processed to recover
the transmitted data streams. RX channel/data processor 356
performs processing complementary to that performed by TX data
processor 314 and TX channel processor 320 and provides decoded
data to a data sink 360. The processing by receiver system 350 is
described in further detail below.
MIMO Transmitter Systems
FIG. 4A is a block diagram of a MIMO transmitter system 310a, which
is capable of processing data in accordance with an embodiment of
the invention. Transmitter system 310a is one embodiment of the
transmitter portion of system 310 in FIG. 3. System 310a includes
(1) a TX data processor 314a that receives and processes
information bits to provide weighted modulation symbols and (2) a
TX channel processor 320a that demultiplexes the modulation symbols
for the selected transmission channels.
In the embodiment shown in FIG. 4A, TX data processor 314a includes
an encoder 412, a channel interleaver 414, a puncturer 416, a
symbol mapping element 418, and a symbol weighting element 420.
Encoder 412 receives the aggregate information bits to be
transmitted and encodes the received bits in accordance with a
particular encoding scheme to provide coded bits. Channel
interleaver 414 interleaves the coded bits based on a particular
interleaving scheme to provide diversity. Puncturer 416 punctures
(i.e., deletes) zero or more of the interleaved coded bits to
provide the desired number of coded bits. Symbol mapping element
418 maps the unpunctured bits into modulation symbols for the
selected transmission channels. And symbol weighting element 420
weighs the modulation symbols for each selected transmission
channel to provide weighted modulation symbols. The weight used for
each selected transmission channel may be determined based on that
channel's achieved SNR, as described above.
Pilot data (e.g., data of known pattern) may also be encoded and
multiplexed with the processed information bits. The processed
pilot data may be transmitted (e.g., in a time division multiplexed
(TDM) manner) in a subset or all of the selected transmission
channels, or in a subset or all of the available transmission
channels. The pilot data may be used at the receiver to perform
channel estimation, as described below.
As shown in FIG. 4A, the data encoding, interleaving, and
puncturing may be achieved based on one or more coding control
signals, which identify the specific coding, interleaving, and
puncturing schemes to be used. The symbol mapping may be achieved
based on a modulation control signal that identifies the specific
modulation scheme to be used. And the symbol weighting may be
achieved based on weights provided for the selected transmission
channels.
In one coding and modulation scheme, the coding is achieved by
using a fixed base code and adjusting the puncturing to achieve the
desired code rate, as supported by the SNR of the selected
transmission channels. The base code may be a Turbo code, a
convolutional code, a concatenated code, or some other code. The
base code may also be of a particular rate (e.g., a rate 1/3 code).
For this scheme, the puncturing may be performed after the channel
interleaving to achieve the desired code rate for the selected
transmission channels.
Symbol mapping element 416 can be designed to group sets of
unpunctured bits to form non-binary symbols, and to map each
non-binary symbol into a point in a signal constellation
corresponding to the modulation scheme selected for use for the
selected transmission channels. The modulation scheme may be QPSK,
M-PSK, M-QAM, or some other scheme. Each mapped signal point
corresponds to a modulation symbol.
The encoding, interleaving, puncturing, and symbol mapping at
transmitter system 310a can be performed based on numerous schemes.
One specific scheme is described in the aforementioned U.S. patent
application Ser. No. 09/776,075.
The number of information bits that may be transmitted for each
modulation symbol for a particular level of performance (e.g., one
percent packet error rate or PER) is dependent on the received SNR.
Thus, the coding and modulation scheme for the selected
transmission channels may be determined based on the
characteristics of the channels (e.g., the channel gains, received
SNRs, or some other information). The channel interleaving may also
be adjusted based on the coding control signal.
Table 1 lists various combinations of coding rate and modulation
scheme that may be used for a number of received SNR ranges. The
supported bit rate for each transmission channel may be achieved
using any one of a number of possible combinations of coding rate
and modulation scheme. For example, one information bit per
modulation symbol may be achieved using (1) a coding rate of 1/2
and QPSK modulation, (2) a coding rate of 1/3 and 8-PSK modulation,
(3) a coding rate of 1/4 and 16-QAM, or some other combination of
coding rate and modulation scheme. In Table 1, QPSK, 16-QAM, and
64-QAM are used for the listed SNR ranges. Other modulation schemes
such as 8-PSK, 32-QAM, 128-QAM, and so on, may also be used and are
within the scope of the invention.
TABLE-US-00001 TABLE 1 Received SNR # of Information Modulation #
of Coded Coding Range Bits/Symbol Symbol Bits/Symbol Rate 1.5 4.4 1
QPSK 2 1/2 4.4 6.4 1.5 QPSK 2 3/4 6.4 8.35 2 16-QAM 4 1/2 8.35 10.4
2.5 16-QAM 4 5/8 10.4 12.3 3 16-QAM 4 3/4 12.3 14.15 3.5 64-QAM 6
7/12 14.15 15.55 4 64-QAM 6 2/3 15.55 17.35 4.5 64-QAM 6 3/4
>17.35 5 64-QAM 6 5/6
The weighted modulation symbols from TX data processor 314a are
provided to TX channel processor 320a, which is one embodiment of
TX channel processor 320 in FIG. 3. Within TX channel processor
320a, a demultiplexer 424 receives and demultiplexes the weighted
modulation symbol into a number of modulation symbol streams, one
stream for each transmission channel selected to transmit the
modulation symbols. Each modulation symbol stream is provided to a
respective modulator 322. If OFDM is employed, the weighted
modulation symbols at each time slot for all selected frequency
subchannels of each transmit antenna are combined into a weighted
modulation symbol vector. Each modulator 322 converts the weighted
modulation symbols (for a system without OFDM) or the weighted
modulation symbol vectors (for a system with OFDM) into an analog
signal, and further amplifies, filters, quadrature modulates, and
upconverts the signal to generate a modulated signal suitable for
transmission over the wireless link.
FIG. 4B is a block diagram of a MIMO transmitter system 310b, which
is capable of processing data in accordance with another embodiment
of the invention. Transmitter system 310b is another embodiment of
the transmitter portion of system 310 in FIG. 3 and includes a TX
data processor 314b and a TX channel processor 320b.
In the embodiment shown in FIG. 4B, TX data processor 314b includes
encoder 412, channel interleaver 414, symbol mapping element 418,
and symbol weighting element 420. Encoder 412 receives and encodes
the aggregate information bits in accordance with a particular
encoding scheme to provide coded bits. The coding may be achieved
based on a particular code and code rate selected by controller
334, as identified by the coding control signals. Channel
interleaver 414 interleaves the coded bits, and symbol mapping
element 418 maps the interleaved bits into modulation symbols for
the selected transmission channels. Symbol weighting element 420
weighs the modulation symbols for each selected transmission
channel based on a respective weight to provide weighted modulation
symbols.
In the embodiment shown in FIG. 4B, transmitter system 310b is
capable of preconditioning the weighted modulation symbols based on
full CSI. Within TX channel processor 320b, a channel MIMO
processor 422 demultiplexes the weighted modulation symbols into a
number of (up to N.sub.C) weighted modulation symbol streams, one
stream for each spatial subchannel (i.e., eigenmode) used to
transmit the modulation symbols. For full-CSI processing, channel
MIMO processor 422 preconditions the (up to N.sub.C) weighted
modulation symbols at each time slot to generate N.sub.T
preconditioned modulation symbols, as follows:
.times..times..times..times..times..times..times. ##EQU00032##
where b.sub.1, b.sub.2, . . . b.sub.N.sub.C are respectively the
weighted modulation symbols for spatial subchannels 1, 2, . . .
N.sub.C; e.sub.ij are elements of an eigenvector matrix E related
to the transmission characteristics from the transmit antennas to
the receive antennas; and x.sub.1, x.sub.2, . . . x.sub.N.sub.T are
the preconditioned modulation symbols, which can be expressed as:
x.sub.1=b.sub.1e.sub.11+b.sub.2e.sub.12+ . . .
+b.sub.N.sub.Ce.sub.1N.sub.C,
x.sub.2=b.sub.1e.sub.21+b.sub.2e.sub.22+ . . .
+b.sub.N.sub.Ce.sub.2N.sub.C, and
x.sub.N.sub.T=b.sub.1e.sub.N.sub.T.sub.1+b.sub.2e.sub.N.sub.T.sub.2+
. . . +b.sub.N.sub.Ce.sub.N.sub.T.sub.N.sub.C. The eigenvector
matrix E may be computed by the transmitter or is provided to the
transmitter by the receiver. The elements of the matrix E are also
taken into account in determining the effective channel gains
H(j,k).
For full-CSI processing, each preconditioned modulation symbol,
x.sub.i, for a particular transmit antenna represents a linear
combination of the weighted modulation symbols for up to N.sub.C
spatial subchannels. For each time slot, the (up to) N.sub.T
preconditioned modulation symbols generated by channel MIMO
processor 422 are demultiplexed by demultiplexer 424 and provided
to (up to) N.sub.T modulators 322. Each modulator 322 converts the
preconditioned modulation symbols (for a system without OFDM) or
the preconditioned modulation symbol vectors (for a system with
OFDM) into a modulated signal suitable for transmission over the
wireless link.
FIG. 4C is a block diagram of a MIMO transmitter system 310c, which
utilizes OFDM and is capable of processing data in accordance with
yet another embodiment of the invention. Transmitter system 310c is
another embodiment of the transmitter portion of system 310 in FIG.
3 and includes a TX data processor 314c and a TX channel processor
320c. TX data processor 314c may be operated to independently code
and modulate each group of transmission channels based on a
particular coding and modulation scheme selected for the group.
Each group may correspond to one transmit antenna and the
transmission channels in each group may correspond to the frequency
subchannels for the transmit antenna.
In the embodiment shown in FIG. 4C, TX data processor 314c includes
a number of spatial subchannel data processor 410a through 410t,
one data processor 410 for each group of transmission channels to
be independently coded and modulated. Each data processor 410
includes encoder 412, channel interleaver 414, symbol mapping
element 418, and symbol weighting element 420. These elements of
data processor 410 operate to encode the information bits for a
group being processed by the data processor, interleave the coded
bits, map the interleaved bits to generated modulation symbols, and
weight the modulation symbols for each selected transmission
channel within the group. As shown in FIG. 4C, the coding and
modulation control and the weights may be specifically provided for
each group.
The weighted modulation symbols from each data processor 410 are
provided to a respective combiner 434 within TX channel processor
320c, which combines the weighted modulation symbols for a
particular transmit antenna. If each group includes the selected
frequency subchannels for a particular transmit antenna, then
combiner 434 combines the weighted modulation symbols for the
selected frequency subchannels to form a modulation symbol vector
for each transmission channel, which is then provided to a
respective modulator 322. The processing by each modulator 322 to
generate a modulated signal is described below.
FIG. 4D is a block diagram of a MIMO transmitter system 310d, which
also utilizes OFDM and is capable of processing data in accordance
with yet another embodiment of the invention. In this embodiment,
the transmission channels for each frequency subchannel may be
independently processed. Within a TX data processor 314c, the
information bits to be transmitted are demultiplexed by a
demultiplexer 428 into a number of (up to N.sub.L) frequency
subchannel data streams, one stream for each of the frequency
subchannels to be used for data transmission. Each frequency
subchannel data stream is provided to a respective frequency
subchannel data processor 430.
Each data processor 430 processes data for a respective frequency
subchannel of the OFDM system. Each data processor 430 may be
implemented similar to TX data processor 314a in FIG. 4A, TX data
processor 314b shown in FIG. 4B, or with some other design. In one
embodiment, data processor 430 demultiplexes the frequency
subchannel data stream into a number of data substreams, one data
substream for each spatial subchannel selected for use for the
frequency subchannel. Each data substream is then encoded,
interleaved, symbol mapped, and weighted to generate weighted
modulation symbols for the data substream. The coding and
modulation for each frequency subchannel data stream or each data
substream may be adjusted based on the coding and modulation
control signals and the weighting may be performed based on the
weights. Each data processor 430 thus provides up to N.sub.C
weighted modulation symbol streams for up to N.sub.C spatial
subchannels selected for use for the frequency subchannel.
For a MIMO system utilizing OFDM, the modulation symbols may be
transmitted on multiple frequency subchannels and from multiple
transmit antennas. Within a MIMO processor 320d, the up to N.sub.C
modulation symbol streams from each data processor 430 are provided
to a respective subchannel spatial processor 432, which processes
the received modulation symbols based on the channel control and/or
the available CSI. Each spatial processor 432 may simply implement
a demultiplexer (such as that shown in FIG. 4A) if full-CSI
processing is not performed, or may implement a channel MIMO
processor followed by a demultiplexer (such as that shown in FIG.
4B) if full-CSI processing is performed. For a MIMO system
utilizing OFDM, the full-CSI processing (i.e., preconditioning) may
be performed on each frequency subchannel.
Each subchannel spatial processor 432 demultiplexes the up to
N.sub.C modulation symbols for each time slot into up to N.sub.T
modulation symbols for the transmit antennas selected for use for
that frequency subchannel. For each transmit antenna, a combiner
434 receives the modulation symbols for up to N.sub.L frequency
subchannels selected for use for that transmit antenna, combines
the symbols for each time slot into a modulation symbol vector V,
and provides the modulation symbol vector to the next processing
stage (i.e., a respective modulator 322).
MIMO processor 320d thus receives and processes the modulation
symbols to provide up to N.sub.T modulation symbol vectors, V.sub.1
through V.sub.Nt, one modulation symbol vector for each transmit
antenna selected for use for data transmission. Each modulation
symbol vector V covers a single time slot, and each element of the
modulation symbol vector V is associated with a specific frequency
subchannel having a unique subcarrier on which the modulation
symbol is conveyed.
FIG. 4D also shows an embodiment of modulator 322 for OFDM. The
modulation symbol vectors V.sub.1 through V.sub.Nt from MIMO
processor 320c are provided to modulators 322a through 322t,
respectively. In the embodiment shown in FIG. 4D, each modulator
322 includes an inverse Fast Fourier Transform (IFFT) 440, a cyclic
prefix generator 442, and an upconverter 444.
IFFT 440 converts each received modulation symbol vector into its
time-domain representation (which is referred to as an OFDM symbol)
using IFFT. IFFT 440 can be designed to perform the IFFT on any
number of frequency subchannels (e.g., 8, 16, 32, and so on). In an
embodiment, for each modulation symbol vector converted to an OFDM
symbol, cyclic prefix generator 442 repeats a portion of the
time-domain representation of the OFDM symbol to form a
"transmission symbol" for a specific transmit antenna. The cyclic
prefix insures that the transmission symbol retains its orthogonal
properties in the presence of multipath delay spread, thereby
improving performance against deleterious path effects. The
implementation of IFFT 440 and cyclic prefix generator 442 is known
in the art and not described in detail herein.
The time-domain representations from each cyclic prefix generator
442 (i.e., the transmission symbols for each antenna) are then
processed (e.g., converted into an analog signal, modulated,
amplified, and filtered) by upconverter 444 to generate a modulated
signal, which is then transmitted from a respective antenna
324.
OFDM modulation is described in further detail in a paper entitled
"Multicarrier Modulation for Data Transmission An Idea Whose Time
Has Come," by John A. C. Bingham, IEEE Communications Magazine, May
1990, which is incorporated herein by reference.
FIGS. 4A through 4D show four designs of a MIMO transmitter capable
of implementing various aspects and embodiments of the invention.
The invention may also be practiced in an OFDM system that does not
utilize MIMO. In this case, the available transmission channels
correspond to the frequency subchannels of the OFDM system.
Numerous other transmitter designs are also capable of implementing
various inventive techniques described herein, and these designs
are also within the scope of the invention. Some of these
transmitter designs are described in further detail in the
following patent applications, which are all assigned to the
assignee of the present application and incorporated herein by
reference: U.S. patent application Ser. No. 09/776,075, described
above; U.S. patent application Ser. No. 09/532,492, entitled "HIGH
EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING
MULTI-CARRIER MODULATION," filed Mar. 22, 2000; U.S. patent
application Ser. No. 09/826,481, "METHOD AND APPARATUS FOR
UTILIZING CHANNEL STATE INFORMATION IN A WIRELESS COMMUNICATION
SYSTEM," filed Mar. 23, 2001; and U.S. patent application Ser. No.
09/854,235, entitled "METHOD AND APPARATUS FOR PROCESSING DATA IN A
MULTIPLE-INPUT MULTIPLE-OUTPUT (MIMO) COMMUNICATION SYSTEM
UTILIZING CHANNEL STATE INFORMATION," filed May 11, 2001. These
patent applications also describe MIMO processing and CSI
processing in further detail.
In general, transmitter system 310 codes and modulates data for all
selected transmission channels (or all selected transmission
channels within each group) based a particular common coding and
modulation scheme. The modulation symbols are further weighted by
weights assigned to the selected transmission channels such that
the desired level of performance is achieved at the receiver. The
techniques described herein are applicable for multiple parallel
transmission channels supported by MIMO, OFDM, or any other
communication scheme (e.g., a CDMA scheme) capable of supporting
multiple parallel transmission channels.
FIG. 4C shows an embodiment wherein the data for each transmit
antenna may be coded and modulated separately based on a coding and
modulation scheme selected for that transmit antenna. Analogously,
FIG. 4D shows an embodiment wherein the data for each frequency
subchannel may be coded and modulated separately based on a coding
and modulation scheme selected for that frequency subchannel. In
general, all available transmission channels (e.g., all spatial
subchannels of all frequency subchannels) may be segregated into
any number of groups of any type, and each group may include any
number of transmission channels. For example, each group may
include spatial subchannels, frequency subchannels, or subchannels
in both domains.
MIMO Receiver Systems
FIG. 5 is a block diagram of a MIMO receiver system 350a capable of
receiving data in accordance with an embodiment of the invention.
Receiver system 350a is one specific embodiment of receiver system
350 in FIG. 3 and implements the successive cancellation receiver
processing technique to receive and recover the transmitted
signals. The transmitted signals from (up to) N.sub.T transmit
antennas are received by each of NR antennas 352a through 352r and
routed to a respective demodulator (DEMOD) 354 (which is also
referred to as a front-end processor).
Each demodulator 354 conditions (e.g., filters and amplifies) a
respective received signal, downconverts the conditioned signal to
an intermediate frequency or baseband, and digitizes the
downconverted signal to provide samples. Each demodulator 354 may
further demodulate the samples with a received pilot to generate a
stream of received modulation symbols, which is provided to an RX
channel/data processor 356a.
If OFDM is employed for the data transmission, each demodulator 354
further performs processing complementary to that performed by
modulator 322 shown in FIG. 4D. In this case, each demodulator 354
includes an FFT processor (not shown) that generates transformed
representations of the samples and provides a stream of modulation
symbol vectors. Each vector includes up to N.sub.L modulation
symbols for up to N.sub.L frequency subchannels selected for use,
and one vector is provided for each time slot. For a transmit
processing scheme in which each frequency subchannel is
independently processed (e.g., as shown in FIG. 4D), the modulation
symbol vector streams from the FFT processors of all N.sub.R
demodulators are provided to a demultiplexer (not shown in FIG. 5),
which "channelizes" the modulation symbol vector stream from each
FFT processor into up to N.sub.L modulation symbol streams
corresponding to the number of frequency subchannels used for the
data transmission. The demultiplexer then provides each of up to
N.sub.L modulation symbol streams to a respective RX MIMO/data
processor 356a.
For a MIMO system not utilizing OFDM, one RX MIMO/data processor
356a may be used to process the N.sub.R modulation symbol streams
from the N.sub.R received antennas. And for a MIMO system utilizing
OFDM, one RX MIMO/data processor 356a may be used to process the
set of N.sub.R modulation symbol streams from the N.sub.R received
antennas for each of up to N.sub.L frequency subchannels used for
data transmission. Alternatively, a single RX channel/data
processor 356a may be used to separately process the set of
modulation symbol streams associated with each frequency
subchannel.
In the embodiment shown in FIG. 5, RX channel/data processor 356a
(which is one embodiment of RX channel/data processor 356 in FIG.
3) includes a number of successive (i.e., cascaded) receiver
processing stages 510, one stage for each of the transmitted data
streams to be recovered by receiver system 350a. In one transmit
processing scheme, selective channel inversion is applied to all
available transmission channels. In this case, the selected
transmission channels may be used to transmit one or more data
streams, each of which may be independently coded with the common
coding scheme. In another transmit processing scheme, selective
channel inversion is applied separately to each transmit antenna.
In this case, the selected transmission channels for each transmit
antenna may be used to transmit one or more data streams, each of
which may be independently coded with the coding scheme selected
for that transmit antenna. In general, if one data stream is
independently coded and transmitted on each spatial subchannel,
then the successive cancellation receiver processing technique may
be used to recover the transmitted data streams. For clarity, RX
channel/data processor 356a is described for an embodiment wherein
one data stream is independently coded and transmitted on each
spatial subchannel of a given frequency subchannel being processed
data processor 356a.
Each receiver processing stage 510 (except for the last stage 510n)
includes a channel MIMO/data processor 520 coupled to an
interference canceller 530, and the last stage 510n includes only
channel MIMO/data processor 520n. For the first receiver processing
stage 510a, channel MIMO/data processor 520a receives and processes
the N.sub.R modulation symbol streams from demodulators 354a
through 354r to provide a decoded data stream for the first
transmission channel (or the first transmitted signal). And for
each of the second through last stages 510b through 510n, channel
MIMO/data processor 520 for that stage receives and processes the
N.sub.R modified symbol streams from the interference canceller 520
in the preceding stage to derive a decoded data stream for the
transmission channel being processed by that stage. Each channel
MIMO/data processor 520 further provides CSI (e.g., the received
SNR) for the associated transmission channel.
For the first receiver processing stage 510a, interference
canceller 530a receives the N.sub.R modulation symbol streams from
all N.sub.R demodulators 354. And for each of the second through
second-to-last stages, interference canceller 530 receives the
N.sub.R modified symbol streams from the interference canceller in
the preceding stage. Each interference canceller 530 also receives
the decoded data stream from channel MIMO/data processor 520 within
the same stage, and performs the processing (e.g., coding,
interleaving, modulation, channel response, and so on) to derive
N.sub.R remodulated symbol streams that are estimates of the
interference components of the received modulation symbol streams
due to this decoded data stream. The remodulated symbol streams are
then subtracted from the received modulation symbol streams to
derive N.sub.R modified symbol streams that include all but the
subtracted (i.e., canceled) interference components. The N.sub.R
modified symbol streams are then provided to the next stage.
In FIG. 5, a controller 540 is shown coupled to RX channel/data
processor 356a and may be used to direct various steps in the
successive cancellation receiver processing performed by processor
356a.
FIG. 5 shows a receiver structure that may be used in a
straightforward manner when each data stream is transmitted over a
respective transmit antenna (i.e., one data stream corresponding to
each transmitted signal). In this case, each receiver processing
stage 510 may be operated to recover one of the transmitted signals
targeted for receiver system 350a and provide the decoded data
stream corresponding to the recovered transmitted signal.
For some other transmit processing schemes, a data stream may be
transmitted over multiple transmit antennas, frequency subchannels,
and/or time intervals to provide spatial, frequency, and time
diversity, respectively. For these schemes, the receiver processing
initially derives a received modulation symbol stream for the
signal transmitted on each transmit antenna of each frequency
subchannel. Modulation symbols for multiple transmit antennas,
frequency subchannels, and/or time intervals may then be combined
in a complementary manner as the demultiplexing performed at the
transmitter system. The stream of combined modulation symbols is
then processed to provide the corresponding decoded data
stream.
FIG. 6A is a block diagram of an embodiment of channel MIMO/data
processor 520x, which is one embodiment of channel MIMO/data
processor 520 in FIG. 5. In this embodiment, channel MIMO/data
processor 520x includes a spatial/space-time processor 610, a CSI
processor 612, a selector 614, a demodulation element 618, a
de-interleaver 618, and a decoder 620.
Spatial/space-time processor 610 performs linear spatial processing
on the N.sub.R received signals for a non-dispersive MIMO channel
(i.e., with flat fading) or space-time processing on the N.sub.R
received signals for a dispersive MIMO channel (i.e., with
frequency selective fading). The spatial processing may be achieved
using linear spatial processing techniques such as a channel
correlation matrix inversion (CCMI) technique, a minimum mean
square error (MMSE) technique, and others. These techniques may be
used to null out the undesired signals or to maximize the received
SNR of each of the constituent signals in the presence of noise and
interference from the other signals. The space-time processing may
be achieved using linear space-time processing techniques such as a
MMSE linear equalizer (MMSE-LE), a decision feedback equalizer
(DFE), a maximum-likelihood sequence estimator (MLSE), and others.
The CCMI, MMSE, MMSE-LE, and DFE techniques are described in
further detail in the aforementioned U.S. patent application Ser.
No. 09/854,235. The DFE and MLSE techniques are also described in
further detail by S. L. Ariyavistakul et al. in a paper entitled
"Optimum Space-Time Processors with Dispersive Interference:
Unified Analysis and Required Filter Span," IEEE Trans. on
Communication, Vol. 7, No. 7, July 1999, and incorporated herein by
reference.
CSI processor 612 determines the CSI for each of the transmission
channels used for data transmission. For example, CSI processor 612
may estimate a noise covariance matrix based on the received pilot
signals and then compute the SNR of the k-th transmission channel
used for the data stream to be decoded. The SNR may be estimated
similar to conventional pilot assisted single and multi-carrier
systems, as is known in the art. The SNR for all of the
transmission channels used for data transmission may comprise the
CSI that is reported back to the transmitter system. CSI processor
612 may further provide to selector 614 a control signal that
identifies the particular data stream to be recovered by this
receiver processing stage.
Selector 614 receives a number of symbol streams from
spatial/space-time processor 610 and extracts the symbol stream
corresponding to the data stream to be decoded, as indicated by the
control signal from CSI processor 612. The extracted stream of
modulation symbols is then provided to a demodulation element
614.
For the embodiment shown in FIG. 6A in which the data stream for
each transmission channel is independently coded and modulated
based on the common coding and modulation scheme, the recovered
modulation symbols for the selected transmission channel are
demodulated in accordance with a demodulation scheme (e.g., M-PSK,
M-QAM) that is complementary to the common modulation scheme used
for the transmission channel. The demodulated data from
demodulation element 616 is then de-interleaved by a de-interleaver
618 in a complementary manner to that performed by channel
interleaver 614, and the de-interleaved data is further decoded by
a decoder 620 in a complementary manner to that performed by
encoder 612. For example, a Turbo decoder or a Viterbi decoder may
be used for decoder 620 if Turbo or convolutional coding,
respectively, is performed at the transmitter system. The decoded
data stream from decoder 620 represents an estimate of the
transmitted data stream being recovered.
FIG. 6B is a block diagram of an interference canceller 530x, which
is one embodiment of interference canceller 530 in FIG. 5. Within
interference canceller 530x, the decoded data stream from the
channel MIMO/data processor 520 within the same stage is
re-encoded, interleaved, and re-modulated by a channel data
processor 628 to provide remodulated symbols, which are estimates
of the modulation symbols at the transmitter system prior to the
MIMO processing and channel distortion. Channel data processor 628
performs the same processing (e.g., encoding, interleaving, and
modulation) as that performed at the transmitter system for the
data stream. The remodulated symbols are then provided to a channel
simulator 630, which processes the symbols with the estimated
channel response to provide an estimate, .sup.k, of the
interference due the decoded data stream. The channel response
estimate may be derived based on the pilot and/or data transmitted
by the transmitter system and in accordance with the techniques
described in the aforementioned U.S. patent application Ser. No.
09/854,235.
The N.sub.R elements in the interference vector .sup.k correspond
to the component of the received signal at each of the N.sub.R
receive antennas due to symbol stream transmitted on the k-th
transmit antenna. Each element of the vector represents an
estimated component due to the decoded data stream in the
corresponding received modulation symbol stream. These components
are interference to the remaining (not yet detected) transmitted
signals in the N.sub.R received modulation symbol streams (i.e.,
the vector r.sup.k), and are subtracted (i.e., canceled) from the
received signal vector r.sup.k by a summer 632 to provide a
modified vector r.sup.k+1 having the components from the decoded
data stream removed. The modified vector r.sup.k+1 is provided as
the input vector to the next receiver processing stage, as shown in
FIG. 5.
Various aspects of the successive cancellation receiver processing
are described in further detail in the aforementioned U.S. patent
application Ser. No. 09/854,235.
FIG. 7 is a block diagram of a MIMO receiver system 350b capable of
receiving data in accordance with another embodiment of the
invention. The transmitted signals from (up to) N.sub.T transmit
antennas are received by each of N.sub.R antennas 352a through 352r
and routed to a respective demodulator 354. Each demodulator 354
conditions, processes, and digitizes a respective received signal
to provide samples, which are provided to a RX MIMO/data processor
356b.
Within RX MIMO/data processor 356b, the samples for each receive
antenna are provided to a respective FFT processor 710, which
generates transformed representations of the received samples and
provides a respective stream of modulation symbol vectors. The
streams of modulation symbol vector from FFT processors 710a
through 710r are then provided to a processor 720. Processor 720
channelizes the stream of modulation symbol vectors from each FFT
processor 710 into a number of up to N.sub.L subchannel symbol
streams. Processor 720 may further perform spatial processing or
space-time processing on the subchannel symbol streams to provide
post-processed modulation symbols.
For each data stream transmitted over multiple frequency
subchannels and/or multiple spatial subchannels, processor 720
further combines the modulation symbols for all frequency and
spatial subchannels used for transmitting the data stream into one
post-processed modulation symbol stream, which is then provided to
a data stream processor 730. Each data stream processor 730
performs demodulation, de-interleaving, and decoding complementary
to that performed on the data stream at the transmitter unit and
provides a respective decoded data stream.
Receiver systems that employ the successive cancellation receiver
processing technique and those that do not employ the successive
cancellation receiver processing technique may be used to receive,
process, and recover the transmitted data streams. Some receiver
systems capable of processing signals received over multiple
transmission channels are described in the aforementioned U.S.
patent application Ser. Nos. 09/776,075 and 09/826,481, and U.S.
patent application Ser. No. 09/532,492, entitled "HIGH EFFICIENCY,
HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTI-CARRIER
MODULATION," filed Mar. 30, 2000, assigned to the assignee of the
present invention and incorporated herein by reference.
Obtaining CSI for the Transmitter System
For simplicity, various aspects and embodiments of the invention
have been described wherein the CSI comprises SNR. In general, the
CSI may comprise any type of information that is indicative of the
characteristics of the communication link. Various types of
information may be provided as CSI, some examples of which are
described below.
In one embodiment, the CSI comprises SNR, which is derived as the
ratio of the signal power over the noise plus interference power.
The SNR is typically estimated and provided for each transmission
channel used for data transmission (e.g., each transmit data
stream), although an aggregate SNR may also be provided for a
number of transmission channels. The SNR estimate may be quantized
to a value having a particular number of bits. In one embodiment,
the SNR estimate is mapped to an SNR index, e.g., using a look-up
table.
In another embodiment, the CSI comprises power control information
for each spatial subchannel of each frequency subchannel. The power
control information may include a single bit for each transmission
channel to indicate a request for either more power or less power,
or it may include multiple bits to indicate the magnitude of the
change of power level requested. In this embodiment, the
transmitter system may make use of the power control information
fed back from the receiver systems to determine which transmission
channels to select, and what power to use for each transmission
channel.
In yet another embodiment, the CSI comprises signal power and
interference plus noise power. These two components may be
separately derived and provided for each transmission channel used
for data transmission.
In yet another embodiment, the CSI comprises signal power,
interference power, and noise power. These three components may be
derived and provided for each transmission channel used for data
transmission.
In yet another embodiment, the CSI comprises signal-to-noise ratio
plus a list of interference powers for each observable interference
term. This information may be derived and provided for each
transmission channel used for data transmission.
In yet another embodiment, the CSI comprises signal components in a
matrix form (e.g., N.sub.T.times.N.sub.R complex entries for all
transmit-receive antenna pairs) and the noise plus interference
components in matrix form (e.g., N.sub.T.times.N.sub.R complex
entries). The transmitter system may then properly combine the
signal components and the noise plus interference components for
the appropriate transmit-receive antenna pairs to derive the
quality for each transmission channel used for data transmission
(e.g., the post-processed SNR for each transmitted data stream, as
received at the receiver systems).
In yet another embodiment, the CSI comprises a data rate indicator
for each transmit data stream. The quality of a transmission
channel to be used for data transmission may be determined
initially (e.g., based on the SNR estimated for the transmission
channel) and a data rate corresponding to the determined channel
quality may then be identified (e.g., based on a look-up table).
The identified data rate is indicative of the maximum data rate
that may be transmitted on the transmission channel for the
required level of performance. The data rate is then mapped to and
represented by a data rate indicator (DRI), which can be
efficiently coded. For example, if (up to) seven possible data
rates are supported by the transmitter system for each transmit
antenna, then a 3-bit value may be used to represent the DRI where,
e.g., a zero may indicate a data rate of zero (i.e., don't use the
transmit antenna) and 1 through 7 may be used to indicate seven
different data rates. In a typical implementation, the quality
measurements (e.g., SNR estimates) are mapped directly to the DRI
based on, e.g., a look-up table.
In yet another embodiment, the CSI comprises an indication of the
particular processing scheme to be used at the transmitter system
for each transmit data stream. In this embodiment, the indicator
may identify the particular coding scheme and the particular
modulation scheme to be used for the transmit data stream such that
the desired level of performance is achieved.
In yet another embodiment, the CSI comprises a differential
indicator for a particular measure of quality for a transmission
channel. Initially, the SNR or DRI or some other quality
measurement for the transmission channel is determined and reported
as a reference measurement value. Thereafter, monitoring of the
quality of the transmission channel continues, and the difference
between the last reported measurement and the current measurement
is determined. The difference may then be quantized to one or more
bits, and the quantized difference is mapped to and represented by
the differential indicator, which is then reported. The
differential indicator may indicate to increase or decrease the
last reported measurement by a particular step size (or to maintain
the last reported measurement). For example, the differential
indicator may indicate that (1) the observed SNR for a particular
transmission channel has increased or decreased by a particular
step size, or (2) the data rate should be adjusted by a particular
amount, or some other change. The reference measurement may be
transmitted periodically to ensure that errors in the differential
indicators and/or erroneous reception of these indicators do not
accumulate.
In yet another embodiment, the CSI comprises the channel gain for
each available transmission channel, as estimated at the receiver
system based on signals transmitted by the transmitter system.
Other forms of CSI may also be used and are within the scope of the
invention. In general, the CSI includes sufficient information in
whatever form that may be used to (1) select a set of transmission
channels that will result in optimum or near optimum throughput,
(2) determine a weighting factor for each selected transmission
channel that results in equal or near equal received SNRs, and (3)
infer an optimum or near optimum code rate for the selected
transmission channels.
The CSI may be derived based on the signals transmitted from the
transmitter system and received at the receiver systems. In an
embodiment, the CSI is derived based on a pilot reference included
in the transmitted signals. Alternatively or additionally, the CSI
may be derived based on the data included in the transmitted
signals. Although data may be transmitted on only the selected
transmission channels, pilot data may be transmitted on unselected
transmission channels to allow the receiver systems to estimate the
channel characteristics.
In yet another embodiment, the CSI comprises one or more signals
transmitted from the receiver systems to the transmitter system. In
some systems, a degree of correlation may exist between the uplink
and downlink (e.g. time division duplexed (TDD) systems where the
uplink and downlink share the same frequency band in a time
division multiplexed manner). In these systems, the quality of the
uplink may be estimated (to a requisite degree of accuracy) based
on the quality of the downlink, and vice versa, which may be
estimated based on signals (e.g., pilot signals) transmitted from
the receiver systems. The pilot signals would then represent a
means for which the transmitter system could estimate the CSI as
observed at the receiver systems. For this type of CSI, no
reporting of channel characteristics is necessary.
The signal quality may be estimated at the transmitter system based
on various techniques. Some of these techniques are described in
the following patents, which are assigned to the assignee of the
present application and incorporated herein by reference: U.S. Pat.
No. 5,799,005, entitled "SYSTEM AND METHOD FOR DETERMINING RECEIVED
PILOT POWER AND PATH LOSS IN A CDMA COMMUNICATION SYSTEM," issued
Aug. 25, 1998, U.S. Pat. No. 5,903,554, entitled "METHOD AND
APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM
COMMUNICATION SYSTEM," issued May 11, 1999, U.S. Pat. Nos.
5,056,109, and 5,265,119, both entitled "METHOD AND APPARATUS FOR
CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE
SYSTEM," respectively issued Oct. 8, 1991 and Nov. 23, 1993, and
U.S. Pat. No. 6,097,972, entitled "METHOD AND APPARATUS FOR
PROCESSING POWER CONTROL SIGNALS IN CDMA MOBILE TELEPHONE SYSTEM,"
issued Aug. 1, 2000. Methods for estimating a single transmission
channel based on a pilot signal or a data transmission may also be
found in a number of papers available in the art. One such channel
estimation method is described by F. Ling in a paper entitled
"Optimal Reception, Performance Bound, and Cutoff-Rate Analysis of
References-Assisted Coherent CDMA Communications with
Applications," IEEE Transaction On Communication, October 1999.
Various types of information for CSI and various CSI reporting
mechanisms are also described in U.S. patent application Ser. No.
08/963,386, entitled "METHOD AND APPARATUS FOR HIGH RATE PACKET
DATA TRANSMISSION," filed Nov. 3, 1997, assigned to the assignee of
the present application, and in "TIE/EIA/IS-856 cdma2000 High Rate
Packet Data Air Interface Specification", both of which are
incorporated herein by reference.
The CSI may be reported back to the transmitter using various CSI
transmission schemes. For example, the CSI may be sent in full,
differentially, or a combination thereof. In one embodiment, CSI is
reported periodically, and differential updates are sent based on
the prior transmitted CSI. In another embodiment, the CSI is sent
only when there is a change (e.g., if the change exceeds a
particular threshold), which may lower the effective rate of the
feedback channel. As an example, the SNRs may be sent back (e.g.,
differentially) only when they change. For an OFDM system (with or
without MIMO), correlation in the frequency domain may be exploited
to permit reduction in the amount of CSI to be fed back. As an
example for an OFDM system, if the SNR corresponding to a
particular spatial subchannel for M frequency subchannels is the
same, the SNR and the first and last frequency subchannels for
which this condition is true may be reported. Other compression and
feedback channel error recovery techniques to reduce the amount of
data to be fed back for CSI may also be used and are within the
scope of the invention.
Referring back to FIG. 3, the CSI (e.g., the received SNR)
determined by RX channel/data processor 356 is provided to a TX
data processor 362, which processes the CSI and provides processed
data to one or more modulators 354. Modulators 354 further
condition the processed data and transmit the CSI back to
transmitter system 310 via a reverse channel.
At system 310, the transmitted feedback signal is received by
antennas 324, demodulated by demodulators 322, and provided to a RX
data processor 332. RX data processor 332 performs processing
complementary to that performed by TX data processor 362 and
recovers the reported CSI, which is then provided to controller
334.
Controller 334 uses the reported CSI to perform a number of
functions including (1) selecting the set of N.sub.S best available
transmission channels for data transmission, (2) determining the
coding and modulation scheme to be used for data transmission on
the selected transmission channels, and (3) determining the weights
to be used for the selected transmission channels. Controller 334
may select the transmission channels to achieve high throughput or
based on some other performance criteria or metrics, and may
further determine the threshold used to select the transmission
channels, as described above.
The characteristics (e.g., channel gains or received SNRs) of the
transmission channels available for data transmission may be
determined based on various techniques as described above and
provided to the transmitter system. The transmitter system may then
use the information to select the set of N.sub.S best transmission
channels, properly code and modulate the data, and further weight
the modulation symbols.
The techniques described herein may be used for data transmission
on the downlink from a base station to one or more terminals, and
may also be used for data transmission on the uplink from each of
one or more terminals to a base station. For the downlink,
transmitter system 310 in FIGS. 3 and 4A through 4D may represent
part of a base station and receiver system 350 in FIGS. 3, 5, and 6
may represent part of a terminal. And for the uplink, transmitter
system 310 in FIGS. 3 and 4A through 4D may represent part of a
terminal and receiver system 350 in FIGS. 3, 5, and 6 may represent
part of a base station.
The elements of the transmitter and receiver systems may be
implemented with one or more digital signal processors (DSP),
application specific integrated circuits (ASIC), processors,
microprocessors, controllers, microcontrollers, field programmable
gate arrays (FPGA), programmable logic devices, other electronic
units, or any combination thereof. Some of the functions and
processing described herein may also be implemented with software
executed on a processor. Certain aspects of the invention may also
be implemented with a combination of software and hardware. For
example, computations to determine the threshold, .alpha., and to
select transmission channels may be performed based on program
codes executed on a processor (controller 334 in FIG. 3).
Headings are included herein for reference and to aid in the
locating certain sections. These heading are not intended to limit
the scope of the concepts described therein under, and these
concepts may have applicability in other sections throughout the
entire specification.
The previous description of the disclosed embodiments is provided
to enable any person skilled in the art to make or use the present
invention. Various modifications to these embodiments will be
readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *